1. Field of the Invention
This invention relates in general to oil and gas well drilling equipment, and in particular to an apparatus for tensioning a section of casing.
2. Description of the Prior Art
In oil and gas well drilling, the casing will be cemented in the well. In some well installations, the operator will tension the casing above the level of cement, and support the casing in tension at the wellhead. For example in one type of offshore drilling, the level of cement around the casing will be located at or below the sea floor. The casing extends upward to a surface wellhead housing at the drilling or production platform. The surface wellhead will have pressure control equipment. In very deep water, the extension of casing above the sea floor could be a considerable distance.
The section of the casing extending from the level of cement to the surface wellhead housing must be placed in tension. This is handled by pulling upward on the section with a drilling rig, then anchoring the casing in tension at the surface wellhead housing. Various devices to maintain the casing in tension are available.
Under the prior techniques, the amount of tension depends upon the amount of overpull applied by the drilling rig. An operator may have means for calculating in advance how much stretch that he wishes to place in the casing. However, it is difficult to precisely pull and maintain the calculated amount of stretch. Normally, the operator marks the pipe, then pulls it upward for the calculated amount. The operator then attempts to hang the casing in the wellhead housing at the desired point of stretch.